In their laboratory, Dr. Thompson and his assistant Mark Stone had had the cyclists pedal as hard as they could on a stationary bicycle for the equivalent of 4,000 meters, about 2.5 miles. After they had done this on several occasions, the cyclists thought they knew what their limits were.

Then Dr. Thompson asked the cyclists to race against an avatar, a figure of a cyclist on a computer screen in front them. Each rider was shown two avatars. One was himself, moving along a virtual course at the rate he was actually pedaling the stationary bicycle. The other figure was moving at the pace of the cyclist’s own best effort — or so the cyclists were told.

In fact, the second avatar was programmed to ride faster than the cyclist ever had — using 2 percent more power, which translates into a 1 percent increase in speed.

Told to race against what they thought was their own best time, the cyclists ended up matching their avatars on their virtual rides, going significantly faster than they ever had gone before.

While a 2 percent increase in power might seem small, it is enough to make a big difference in a competitive event that lasts four to five minutes, like cycling for 4,000 meters. At the elite level in sports, a 1 percent increase in speed can determine whether an athlete places in a race or comes in somewhere farther back in the pack.

The improved times observed in his experiment, said Dr. Thompson, are “not just day-to-day variability, but a true change in performance.” And they give rise to some perplexing questions.

What limits how fast a person can run or swim or cycle or row? Is it just the body — do fatigued muscles just give out at a certain point? Or is the limit set by a mysterious “central governor” in the brain, as Timothy Noakes, professor of exercise and sports science at the University of Cape Town in South Africa, has called it, that determines pacing and effort and, ultimately, performance?

Until recently, exercise physiologists have mostly focused on the muscles, hearts and lungs of athletes, asking whether fatigue comes because the body has reached its limit.

But athletes themselves have long insisted that mental factors are paramount. Roger Bannister, the first runner to break the four-minute mile, once said: “It is the brain, not the heart or lungs that is the critical organ. It’s the brain.”

Now researchers like Dr. Thompson are designing studies to learn more about the brain’s influence over athletic performance.

For example, Jo Corbett, a senior lecturer in applied exercise physiology at the University of Portsmouth in England, wondered how much competition can affect an athlete’s speed. To find out, he asked cyclists to ride as hard and as fast as they could on a stationary bicycle for the equivalent of 2,000 meters. As he rode, each rider was shown an on-screen figure representing the cyclist riding the course.

Then Dr. Corbett and his colleagues told each athlete that he would be racing against another rider hidden behind a screen. The researchers projected two figures on the screen, one the outline of the rider and the other the outline of the competitor.

In fact, the competitor on the screen was a computer-generated image of the athlete himself in his own best attempt to ride those 2,000 meters.

The cyclists rode furiously through the on-screen race. And, as happened in Dr. Thompson’s experiments, the cyclists beat their best times, finishing with a burst of speed that carried them to virtual victory by a significant length.

Dr. Corbett said the extra effort, above and beyond what the athletes had previously demonstrated, seems to come from the anaerobic energy system, one that is limited by the amount of fuel stored in muscle. The brain appears to conserve the body’s limited fuel to a certain degree, not allowing athletes to work too hard.

Compártelo:

Last week, the American sprinter Justin Gatlin showed up at the World Outdoor Track and Field Championships in Daegu, South Korea, with frostbite on his feet. This condition was painful — he told reporters that he had blisters on both heels — but it was also improbable, given that he’d developed the frostbite in Florida in August. But Mr. Gatlin had been sampling one of the newest, trendiest innovations in elite athlete training. He’d gone into a whole-body cryotherapy chamber, and his feet had frozen there.

The American sprinter Justin Gatlin after competing in the men’s 100-meter raceat the International Association of Athletics Federations’ world championships in Daegu, South Korea.

Whole-body cryotherapy is, essentially, ice baths taken to a new and otherworldly level, and it is drawing considerable attention among athletes, both elite and recreational. In the cryotherapy chambers, the ambient temperature is lowered to a numbing minus 110 Celsius or minus 166 Fahrenheit. The chambers were originally intended to treat certain medical conditions, but athletes soon adopted the technology in hopes that supra-subzero temperatures would help them to recover from strenuous workouts more rapidly.

That they would place faith in cold therapy is surprising, given that studies examining the effects of simple ice baths have been, at best, “inconclusive,” said Joseph Costello, a doctoral student in the physical education and sports sciences department at the University of Limerick in Ireland, who is studying the effects of whole-body cryotherapy.

A 2007 study of ice baths found that young men who completed a punishing 90-minute shuttle run and then eased themselves into a frigid bathtub (with the water cooled to 50 degrees Fahrenheit) for 10 minutes reported feeling markedly less sore a few days later than a control group who did not soak. But ice baths did not lower the runners’ levels of creatine kinase, often considered a hallmark of muscle damage. They felt better, but their muscles were almost as damaged as if they hadn’t soaked.

Despite such findings, a growing number of elite soccer players, rugby teams, professional cyclists and track and field athletes in the United States and Europe have eagerly turned to whole-body cryotherapy. Because no agency in the United States or Europe regulates it, it’s impossible to say with any precision how many athletes are currently using the treatment, but researchers like Mr. Costello say the numbers are growing rapidly.

Before entering a cryochamber, users must strip to shorts or a bathing suit, remove all jewelry and don several pairs of gloves, a face mask, a woolly headband and dry socks. Mr. Gatlin neglected that last precaution; his socks were sweaty from a previous workout and froze instantly to his feet. The athletes then move through an acclimatization chamber set to about minus 76 Fahrenheit and from there into the surface-of-the-moon-chilly cryotherapy chamber.

At minus 110 degrees Celsius, whole-body cryotherapy is “colder than any temperature ever experienced or recorded on earth,” Mr. Costello said.

The athletes remain in the chamber for no more than two or three minutes, stamping their feet and waving their arms to retain circulation. A Welsh rugby player described the experience as being in an “evil” sauna, but told British reporters that he believed that the sessions were helping him to recover more quickly from rigorous practices.

The science to support that optimistic appraisal is slim, though. A study by Mr. Costello, published earlier this year in the Scandinavian Journal of Medicine and Science in Sports, found that whole-body cryotherapy did not lessen muscle damage among a group of volunteers who’d completed grueling resistance exercises with their legs before entering the chamber.

Another study, however, published in July in the Public Library of Science One, produced more encouraging results. For it, French researchers recruited a group of trained runners and put them through a simulated 48-minute trail run on a treadmill. The workout was designed to elicit muscle damage and soreness. Afterward, half of the runners entered a whole-body cryotherapy chamber once a day for five days. The rest sat quietly for 30 minutes a day for those five days. Blood was drawn from both groups throughout the experiment.

From the first day onward, the runners who’d entered the chamber showed fewer blood markers of inflammation than the group who had recovered by sitting quietly.

These results suggest that athletes could potentially “save two to three days” of training time compared with forgoing whole-body cryotherapy, François Bieuzen, a professor at the National Institute of Sport, Expertise and Performance in Paris and lead author of the study, wrote in an e-mail. By using the therapy, tired athletes could return to hard training sooner.

But Alan Donnelly, a professor at the University of Limerick and Mr. Costello’s adviser and co-author, is unconvinced. Reducing inflammation, he points out, does not ensure that muscles have recovered. The French researchers did not directly test muscle strength and function after the cryotherapy sessions. So it’s possible that the athletes’ muscles, although less inflamed, were still weak and damaged.

“I just don’t feel that the evidence base for WBC effectiveness is there yet,” Dr. Donnelly said. “If WBC were a clinical treatment or a nutritional aid being put forward for F.D.A. approval, my view is that it would not be approved.”

Such skepticism is not cooling enthusiasm among athletes, however. A cryotherapy chamber that caters to recreational athletes opened in Northern California last month. Its instructional materials caution users to check that all body parts and clothing, including socks, are completely dry before entering the chamber. Frostbite, as Mr. Gatlin discovered, will impede athletic performance. In his signature event, the 100-meter dash, he did not make the finals.